iio: adc: Add round robin ADC driver

iio: adc: Add round robin ADC driver

Round robin ADC (RRADC) driver provides an interface to enable
clients to read ADC values from the RRADC controller. The ADC
values are updated in a round robin sequence among the enabled
channels.

Clients include reading voltage, current and temperature channels
such as battery ID, battery thermistors, skin temperature,
PMI die temperature, charger temperature, USB_IN and DCIN voltage
and current.

Change-Id: I927c0f6a7db654880d951729996a27310f6628cf
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>